The deltoid is sometimes considered one muscle with three sections and sometimes treated as three separate muscles.
Together they form the bulk of your shoulder and are used to raise your arm from your side and flex and extend your arm at the shoulder.
Problems with the deltoid muscles are typically either related to underlying dysfunction in the rotator cuff muscles, chronic overload of the muscles from poor ergnomics and sports activities, or impact trauma to your shoulder.
The relationship between the deltoids and the rotator cuff muscles, especially supraspinatus, is just being fully understood.
Deltoid trigger points are often misdiagnosed as rotator cuff tears, bicipital (biceps) tendonitis, subdeltoid bursitis, subacromial impingment syndrome or C5 radicular pain.
Anterior (Front) Deltoid
The anterior fibers assist the pectoralis major to flex the shoulder. The anterior deltoid also works in tandem with the subscapularis, pecs and lats to internally (medially) rotate the humerus. The activity of these fibers begins before your arm moves to help stabilize the body against the movement. The anterior deltoid also helps flex your arm forward at the shoulder. When your arm is rotated internally the anterior deltoids can also work weakly an adductor. Mechanically, your arm must be medially rotated for the deltoid to have maximum effect in flexion and adduction. This makes the deltoid an antagonist muscle of the pectoralis major and latissimus dorsi. In fact, internal rotation to 45 degrees also increases the activity of your anterior deltoid abduction. However, EMG studies have not shown activity of the anterior deltoid in performing internal rotation.Lateral (Middle) Deltoid
The lateral fibers perform basic shoulder abduction when the shoulder is internally rotated (normal position of your arm at your side), and perform shoulder transverse abduction when the shoulder is externally rotated (thumbs out and up). They are not utilized significantly during strict transverse extension (shoulder internally rotated) such as in rowing movements, which use the posterior fibers. The middle deltoid also assists in flexion of your shoulder, with maximum activity as you approach 90 degrees. When any of the muscles in the rotator cuff become dysfunctional, the middle deltoid is disadvantaged during abduction. However, the front deltoid appears to be able to adapt and take over some additional function in abduction and assist the weaked rotator cuff muscles.Posterior (Rear) Deltoid
The posterior fibers assist the latissimus dorsi to extend your shoulder. Other transverse extensors, the infraspinatus and teres minor, also work in tandem with the posterior deltoid as external (lateral) rotators. These are antagonists to strong internal rotators like the pecs and lats. This ability to extend our arm at the shoulder is a necessary part of many sports. It is also essential for toileting, grooming and dressing activies. Although it appears the posterior deltoid should be able to rotate the able, EMG studies have shown otherwise. Along with the front and middle deltoids, the rear deltoid is active during abduction and elevation of our arm. However, maximum EMG activity of some of the posterior fibers does not occur until the arm is raised to 140 degrees, almost straight up, and this is believed to be a force counter balancing the abduction of the rest of the deltoid fibers.Dynamic Stabilization
During pushing the anterior deltoid is maximally active with help from the middle deltoid. The posterior deltoid is most active when pulling. In throwing the posterior deltoid is active during quick throwing activities, probably to assist deceleration of the shoulder joint. This would suggest that the anterior deltoid is an accelerator in the throwing motion but this has not been widely studied. In freestyle swimming, the front and middle deltoids had the most activity during the early to late recovery phases of the crawl stroke. In swimmers with painful shoulders, it appears that this activity is inhibited. Trigger points can cause this type of inhibition when overloaded. When driving, the raising your hands to the top of wheel activitates the front deltoids with some help from the middle. When the steering wheel is held toward the middle the activity of the muscles is balanced. The rear deltoids where most active when as the torque on the steering wheel increases. An important function of the deltoid in humans is preventing the dislocation of the humeral head when a person carries heavy loads. The function of abduction also means that it would help keep carried objects a safer distance away from the thighs to avoid hitting them, as during a farmer's walk. It also ensures a precise and rapid movement of the shoulder joint needed for hand and arm manipulation. The lateral fibers are in the most efficient position to perform this role, though like basic abduction movements (such as lateral raise) it is assisted by simultaneous co-contraction of anterior/posterior fibers.Structure
Previous studies showed that the insertion of the intramuscular tendons of the deltoid muscle formed three discrete sets of muscle fibers, often referred to as "heads":- The anterior or clavicular fibers arise from most of the front border and upper surface of the outside third of the clavicle. The origin on the front lies next to the outside fibers of the pectoralis major as do the end tendons of both muscles. These muscle fibers are closely related and only a small space prevents the two muscles from forming a continuous muscle mass. The anterior deltoids are commonly called front delts for short.
- Lateral or acromial fibers arise from the superior surface of the acromion process of the scapula. They are commonly called lateral deltoid. This muscle is also called middle delts, outer delts, or side delts for short.
- Posterior or spinal fibers arise from the lower lip of the posterior border of the spine of the scapula. They are commonly called posterior deltoid or rear deltoid (rear delts for short).
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